Final answer:
The dramatic irony in Henrik Ibsen's 'A Doll's House' is that the audience knows Nora wants to leave her marriage before the other characters, fulfilling option (b) about the audience being aware of a secret that the characters are not.
Step-by-step explanation:
Given the context of the drama A Doll's House by Henrik Ibsen, the dramatic irony in the passage is best described by option (b): The audience is aware of a secret, but the characters are not. This is because dramatic irony typically occurs when the audience knows something pivotal that the characters within the play are not privy to. In A Doll's House, the audience is aware of Nora's struggles and her thoughts about leaving her marriage well before the other characters are, especially her husband, Torvald. Due to the limitations of the medium, the play relies heavily on dialogue and the actions of the characters on stage to communicate the narrative and inner thoughts. As a result, when Nora finally reveals her intentions to leave to Torvald, the audience has long been in on her secret, heightening the impact of this revelation and the ensuing drama of her departure.